Fifteen British soldiers a week are being thrown out of the army for taking drugs, including heroin, ecstasy, cannabis and cocaine, figures obtained by The Observer reveal.

Almost 800 troops were discharged last year after failing random drug tests. But, with British forces already stretched in Iraq and Afghanistan, some experts have cast doubt on the long-term viability of the Ministry of Defence's zero-tolerance approach to drugs and its compulsory expulsion policy.

Professor David Nutt, a psychopharmacologist at Bristol University and a member of the government's Advisory Council on the Misuse of Drugs, told The Observer he thought the policy was illogical and misguided. Much of the drug use within the armed forces was 'situational' and recreational, and reflected a trend in society at large.

'The whole issue of drug testing is a very complex one,' Nutt said. 'The army has to ask itself the very serious question of what they're trying to do here. Are they saying drugs are illegal, so if you take them you're not fit to be a soldier? Or are they saying using cocaine or cannabis is interfering with your ability to do your job? If it's the latter, then we need to see the evidence for that.'

He also raised concerns that random drug testing had the perverse effect of making people use more dangerous drugs. 'One of the really big problems is that cannabis lasts in the body for much longer, so people using a class C drug are much more likely to be caught than people using more dangerous drugs like crack or heroin. If someone is intoxicated on cannabis going into battle, then there probably isn't any doubt that it would impair their ability to function. But after combat it's very unclear whether it will do any harm at all. The current approach isn't necessarily the most logical way of dealing with the issue.'

He said alcohol was, in some cases, much more dangerous than some illegal drugs. The use of drugs in the military was complex: 'It's well known that heroin was used widely by troops in the Vietnam war to deal with the craziness. Soldiers used drugs while there, but stopped using when they came home. Similarly in Afghanistan, I think many troops will see cannabis as a locally sanctioned form of relaxation.'

Figures show that 769 soldiers failed random drug tests in 2006.

The latest statistics were released after it emerged that 20 soldiers from the Royal Regiment of Scotland were facing compulsory discharge after testing positive for a range of drugs. Another 26 squaddies were sacked earlier this year.

There had been speculation that soldiers were deliberately trying to fail the strict tests to avoid tours of duty in Iraq and Afghanistan. However, the army has strongly rebuffed such allegations, saying soldiers do not know whether they are going to be tested next week, next year or in five years.

Major Phillip Curtis, an army spokesman, said the policy was crystal clear: 'Drugs are incompatible with military service and won't be tolerated. Whether it's class A or class C, there is no distinction. Some soldiers may see taking drugs as a quick way out, but that would be a tiny minority.'

Most of those who'd taken drugs had probably succumbed to peer pressure. 'The army reflects society. When soldiers go away for the weekend, their drug of choice still tends to be alcohol. But some end up taking drugs instead, or as well. However, the prevalence of drug taking in the armed forces is still significantly less than an equivalent sample in civilian society,' Curtis said.

The renewed evidence of high rates of drug-taking in the forces comes amid long-running concerns that the stresses of warfare are turning many young recruits towards illegal substances.

American defence chiefs have reported historically high rates of suicide among personnel who have fought in Iraq and studies have suggested that many returning troops have severe problems with stress and substance abuse.

Clive Fairweather, a former SAS colonel who works for the Combat Stress charity, which treats ex-service personnel for psychological and addiction problems, said there had been a marked change in the culture both within the army and in society as a whole over the past 10 years.

'Most of the problems a decade ago were related to the army's hard-drinking culture. But that has changed dramatically. Drugs are rife in society, particularly in a certain male age group. Many of these soldiers are from inner-city areas. They go home at the weekends and they not only have lots to drink, but they are also exposed to drugs.'

He said there was a question for the public at large: 'They are paying for soldiers to handle a weapon that can kill. Are they prepared to tolerate soldiers who smoke cannabis but don't take heroin? I think the question has to be asked because the army can't keep haemorrhaging troops at the current rate.'